Radar level gauge systems are in wide use for measuring process variables of products contained in tanks, such as filling level, temperature, pressure etc. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product along a transmission line probe. The transmission line probe is generally arranged vertically from top to bottom of the tank. The electromagnetic signals are subsequently reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
The performance of RLGs, and in particular non-contact RLGs, may be negatively affected by electrically conducting elements or structures in the tank, potentially interfering with the level detection making it less reliable. Such interference can be reduced or avoided by providing some kind of wave guiding structure extending into the tank, thereby ensuring that the emitted and reflected signals have a restricted propagation patterns. Guided wave RLG is typically less affected by interference from structures in the tank, as the signals are more or less restricted to the probe. However, in the case of a single wire transmission line probe (i.e. a transmission line probe with only one single conductor), the propagation field of the signals will have a radial extension within a certain radius around the probe.
In some applications, the tank may be provided with electrically conducting structures (e.g. intermediate decks) having a narrow passage (e.g. having a diameter of 1 m or less) in the operating range of the RLG that the probe needs to pass through. One example of such a structure is an intermediate deck, providing manual access to the tank when it is not in use. Another example is structures providing mechanical strength to the tank. If this passage is more narrow than the radial extension of the propagation field, the power of the signal will be reduced. A power loss of around 5% is typically sufficient to significantly reduce the performance of the GWR.
Conventionally, when a tank exhibits passages that are more narrow that the propagation field of a single wire transmission probe, the probe is coated, e.g. with PTFE, to significantly reduce the radial extension of the propagation field. However, a drawback with this solution is a more expensive probe and also an increase in resistive losses leading to a weaker echo from the surface.